Synchronizing signal generating system for laser scanner

ABSTRACT

A synchronizing signal generating system for a laser scanner comprises a source for emitting a first laser beam for scanning and a second laser beam for synchronization of scans by the first laser beam, a polygonal mirror for causing the first laser beam to scan a medium and for causing the second laser beam to scan a grating which has bright portions and dark portions alternately arranged along a scanning direction of the second laser beam, a converging optical system for converging the second laser beam transmitted through the grating, a light receiving system for receiving the second laser beam converged by the converting optical system and for generating a synchronizing signal, and a circuit for controlling the source responsive to the synchronizing signal so as to synchronize a scan timing of the first laser beam. The converging optical system comprises a plurality of lenses or concave mirrors provided for a length corresponding to a scan length of the second laser beam on the grating.

BACKGROUND OF THE INVENTION

The present invention generally relates to synchronizing signalgenerating systems, and more particularly to a synchronizing signalgenerating system for a laser scanner.

A laser scanner uses a laser beam to write (record) and/or readinformation on and/or from a recording medium. Generally, the laserscanner is provided with a deflector such as a polygonal mirror (orpolygonal scanner) for deflecting a laser beam which is to scan therecording medium. However, it is virtually impossible to keep the scantiming constant for each scan because the rotation of the deflectorcannot be maintained perfectly constant and mirror surfaces of thedeflector cannot be finished to perfect mirror surfaces. For thisreason, a synchronizing signal is required to control the scan timing toan optimum timing.

Conventionally, prior to each scan by the laser beam, the scan issynchronized by detecting the laser beam immediately prior to the scan.However, such a synchronization which simply detects the laser beam atone point prior to each scan is insufficient, because the scanning speedis not perfectly constant due to a deviation in the rotational speed ofthe deflector, a deviation of the characteristic of a fθ-lens from anideal linear characteristic and the like.

Accordingly, methods of more accurately synchronizing the scan wereproposed in Japanese Laid-Open Patent Applications Nos. 54-97050 and60-124938. These methods use a laser beam for recording and anotherlaser beam for scan synchronization.

FIG. 1 shows an essential part of a recording apparatus employing alaser scanner in which the synchronizing signal is basically generatedaccording to such proposed methods. The recording apparatus comprises alaser diode 1 for emitting a laser beam for recording (hereinafterreferred to as a recording beam, a laser diode 2 for emitting a laserbeam which is used for generating a synchronizing clock signal(hereinafter referred to as a synchronizing beam), a polygonal mirror 3,an fθ-lens 4, mirrors 6 and 7, a concave mirror 8, a light receivingelement 9 such as a photodetector, a grating 10, an amplifier 11, ashaping circuit 13, a laser diode driver 15, and an information source17.

The laser beams emitted from the laser diodes 1 and 2 are deflected bythe polygonal mirror and is transmitted through the fθ-lens 4, so thatthe recording beam scans a recording medium 5 such as a photosensitivesheet or drum to record an information and the synchronizing beam scansthe grating 10 by way of the mirrors 6 and 7.

As shown in FIG. 2, the grating 10 comprises minute bright portions 10aand minute dark portions 10b which occur alternately with apredetermined pitch. When the synchronizing beam scans the grating 10and a beam spot SP of the synchronizing beam moves in a direction A, theintensity of the synchronizing beam transmitted through the grating 10becomes modulated depending on the arrangement of the bright and darkportions 10a and 10b. The synchronizing beam transmitted through thegrating 10 is converged by the concave mirror 8 and is directed to thelight receiving element 9 where it is subjected to a photoelectricconversion. The light receiving element 9 outputs a pulse signal whichis passed through the amplifier 11 and the shaping circuit 13, and anoutput pulse signal of the shaping circuit 13 is supplied to the laserdiode driver 15 as a synchronizing clock signal. The laser diode driver15 produces an image clock signal which has a frequency higher frequencythan that of the incoming synchronizing clock signal and is synchronizedto the synchronizing clock signal, and drives the laser diode 1 insynchronism with the image clock signal depending on information dataentered from the information source 17. Since the synchronizing clocksignal is generated based on the synchronizing beam, the driving timingof the laser diode 1 is automatically adjusted even when the rotationalspeed of the polygonal mirror 3 becomes unstable during the recordingoperation. Therefore, the recording operation is carried out with anappropriate scan timing.

The grating 10 itself is known, and is sometimes referred to as a slitor grid scale. The slit scale comprises a light transmitting portion anda non-transmitting portion which occur alternately with a predeterminedpitch.

The problem of the conventional methods of generating the synchronizingsignal is in that the synchronizing beam transmitted through the grating10 is converged and directed to the single light receiving element 9 byuse of the concave mirror 8. In other words, when the scanning distance(width) per scan becomes long, it becomes necessary to use a largeconcave mirror, but such a large concave mirror cannot converge thesynchronizing beam satisfactorily to a small beam spot on the lightreceiving element 9 due to aberration and errors introduced during theproduction of the concave mirror. As a result, it becomes extremelydifficult to generate the synchronizing clock signal with a highaccuracy when such a large concave mirror is used.

The use of a mirror array is proposed in a Japanese Laid-Open PatentApplication No. 60-72473 as a method of eliminating some of the problemsdescribed before. According to this method, the synchronizing beamtransmitted through the grating is converged and directed to a pluralityof light receiving elements by a plurality of concave mirrorsconstituting the mirror array. However, a vapor deposition process isneeded to produce such a mirror array, and the production cost of themirror array is high. In addition, since the plurality of lightreceiving elements are located in an optical path between the gratingand the mirror array, the mirror array must be arranged obliquely, thatis, optical axes of the plurality of concave mirrors of the mirror arraymust lie on a plane oblique to an optical axis of an fθ-lens throughwhich the beam reaches the grating, so as to avoid interference of thebeam directed to the mirror array and the beam directed to each of theplurality of light receiving elements, and the positioning of the mirrorarray is difficult and troublesome to perform.

In addition, scattering of the reflected light occurs at each boundaryportion between two mutually adjacent concave mirrors of the mirrorarray, and there is a decrease in the quantity of light reaching thelight receiving element from the boundary portion. As a result, theoutput of the light receiving element deviates and the synchronizingclock signal becomes unstable, thereby making it impossible to carry outan accurate synchronous detection. In order to ensure the generation ofa stable and accurate synchronizing clock signal, it is essential toprovide a compensation circuit to compensate for the output deviation ofthe light receiving element caused by the scattering of the reflectedlight at the boundary portion, but the use of such a compensationcircuit makes the construction of the laser scanner complex.

In other words, the duty cycle of the synchronizing clock signal becomesunstable at the boundary portion between the two mutually adjacentconcave mirrors of the mirror array. In extreme cases, a signal dropoutoccurs at the boundary portion. Usually, a phase locked loop (PLL)circuit is used to match the phase of the image clock signal with thatof the synchronizing clock signal. The image clock signal is used toenable and disable the recording operation. The PLL circuit comprises aphase comparator for comparing the phases of the synchronizing clocksignal from the shaping circuit and an output signal of a voltagecontrolled oscillator (VCO) which is controlled by an output controlvoltage of the phase comparator, and the output signal of the VCO isused as the image clock signal.

For this reason, the unstable duty cycle of the synchronizing clocksignal and the signal dropout in the synchronizing clock signal causethe PLL circuit to run from the locked state and cause a sudden changein the oscillation frequency of the VCO. In these cases, the image clocksignal becomes unstable and deteriorates the quality of the recordingmade on the recording medium. In terms of the freqency, the change inthe duty cycle of the synchronizing clock signal causes a voltage changein the output control voltage of the phase comparator, and this changein the control voltage causes a frequency change in the image clocksignal. The change in the frequency of the image clock signal appears asmoire and the like on the recording medium and greatly deteriorates thequality of the recording.

On the other hand, a Japanese Laid-Open Patent Application No. 60-75168discloses a method of eliminating the undesirable effects of thescattering of the reflected light at the boundary portion. This methoduses two mirror arrays which are essentially positioned one on top ofthe other. A first mirror array is made up of concave mirrors havingboundary portions which do not coincide with boundary portions ofconcave mirrors constituting a second mirror array. A first group oflight receiving elements are provided to receive reflected lights fromthe first mirror array, and a second group of light receiving elementsare provided to receive reflected lights from the second mirror array.The synchronizing clock signal is derived by adding outputs of the lightreceiving elements in the first and second groups. However, this methodrequires a complex converging optical system, and furthermore, a lightreceiving system for receiving the converged light from the convergingoptical system also becomes complex due to the large number of lightreceiving elements used.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful synchronizing signal generating system for a laserscanner, in which the problems described heretofore are eliminated.

Another and more specific object of the present intention is to providea synchronizing signal generating system for a laser scanner, comprisingfirst means for emitting a first laser beam for scanning and a secondlaser beam for synchronization of scans by the first laser beam, secondmeans for scanning a medium by the first laser beam, third means forscanning a grating by the second laser beam, where the grating hasbright portions and dark portions alternately arranged along a scanningdirection of the second laser beam, a converging optical system forconverging the second laser beam transmitted through the grating, alight receiving system for receiving the second laser beam converged bythe converting optical system and for generating a synchronizing signal,and fourth means for controlling the first means responsive to thesynchronizing signal so as to synchronize a scan timing of the firstlaser beam. The converging optical system comprising a plurality oflenses provided for a length corresponding to a scan length of thesecond laser beam on the grating. According to the synchronizing signalgenerating system of the present invention, the lenses of the convergingoptical system need not be produced with a high precision, and thesynchronizing signal can be generated by use of an inexpensive andsimple arrangement.

Still another object of the present invention is to provide asynchronizing signal generating system further provided with means forpreventing the second laser beam from scanning a junction portionbetween two mutually adjacent lenses of the converging optical system.According to the synchronizing signal generating system of the presentinvention, it is possible to prevent scattering of the second laser beamat the junction between two mutually adjacent lenses of the convergingoptical system, without using a correcting circuit. Hence, it ispossible to generate a stable synchronizing signal. In addition, whenthe lenses are provided with a predetermined relationship to thegrating, the second laser beam is stably converged at the lightreceiving system.

A further object of the present invention is to provide a synchronizingsignal generating system in which the lenses are arranged along ascanning direction of the second laser beam at predetermined intervals,the third means comprises an fθ-lens through which the second laser beamreaches the grating, and the lenses of th converging optical system arearranged within predetermined ranges of the grating so as to includeinflection points of a linearity characteristic of the fθ-lens.According to the synchronizing signal generating system of the presentinvention, it is possible to reduce both the construction and cost ofthe converging optical system and the light receiving system.

Another object of the present invention is to provide a synchronizingsignal generating system which further comprises a signal generatingmeans for generating a pseudo synchronizing signal which is to be usedin place of the synchronizing signal at portions where the synchronizingsignal becomes unstable. According to the synchronizing signalgenerating system of the present invention, it is possible to generatean extremely stable and accurate synchronizing signal.

Still another object of the present invention is to provide asynchronizing signal generating system which further comprises acorrecting circuit for correcting a phase of the synchronizing signal sothat a corrected synchronizing signal has a constant duty cycle.According to the synchronizing signal generating system of the presentinvention, it is possible to generate an extremely stable and accuratesynchronizing signal.

A further object of the present invention is to provide a synchronizingsignal generating system in which the synchronizing signal is generatedduring a scan duration in which the medium scanned by the first laserbeam and no synchronizing signal is generated during a non-scan durationin which no scan of the medium is made by the first laser beam, and thesynchronizing signal generating system further comprises signalgenerating means for generating a pseudo synchronizing signal during thenon-scan duration and selector means for selectively outputting thesynchronizing signal and the pseudo synchronizing signal so as to outputa continuous synchronizing signal, where pseudo synchronizing signal hasa predetermined frequency identical to that of the synchronizing signal.The predetermined frequency may be an integral multiple of a scanningfrequency of the first laser beam with respect to the medium, or thesignal generating means may generate the pseudo synchronizing signalhaving a phase which coincides with that of the synchronizing signal.

Another object of the present invention is to provide a synchronizingsignal generating system for a laser scanner, comprising first means foremitting a first laser beam for scanning and a second laser beam forsynchronization of scans by the first laser beam, second means forscanning a medium by the first laser beam, third means for scanning agrating by the second laser beam, where the grating has bright portionsand dark portions alternately arranged along a scanning direction of thesecond laser beam, a converging optical system for converging the secondlaser beam transmitted through the grating, a light receiving system forreceiving the second laser beam converged by the converting opticalsystem and for generating a synchronizing signal, and fourth means forcontrolling the first means responsive to the synchronizing signal so asto synchronize a scan timing of the first laser beam. The convergingoptical system comprises a mirror array made up of a plurality ofconcave mirrors provided for a length corresponding to a scan length ofthe second laser beam on the grating. A ratio Pm/Pg between a pitch Pmof the concave mirrors and a pitch Pg of the bright and dark portions ofthe grating being set to an integer or slightly greater than an integer,so that the second laser beam transmitted through the grating isprevented from scanning a boundary portion between two mutually adjacentconcave mirrors of the mirror array. According to the synchronizingsignal generating system of the present invention, it is possible togenerate a stable synchronizing signal even at the boundary portionbetween two mutually adjacent concave mirrors of the converging opticalsystem. Signal generating means may be further provided to generate apseudo synchronizing signal which is to be used in place of thesynchronizing signal at portions where the synchronizing signal becomesunstable, so as to ensure the generation of an extremely stable andaccurate synchronizing signal.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 generally shows an essential part of a recording apparatusemploying a laser scanner in which the synchronizing signal is basicallygenerated according to the conventional method;

FIG. 2 is a front view showing an essential part of a grating used inthe recording apparatus shown in FIG. 1;

FIG. 3 generally shows an essential part of a first embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus;

FIG. 4 generally shows an essential part of a second embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus;

FIG. 5 generally shows an essential part of a third embodiment of thesynchronizing signal generating system according to the presentinvention applied to a document reading apparatus;

FIG. 6A is a front view showing a part of the lens array used in thefirst through third embodiments;

FIG. 6B shows a synchronizing clock signal in correspondence with FIG.6A for explaining a signal dropout in the synchronizing clock signal;

FIGS. 7A and 7B are a front view and a perspective view respectivelyshowing a part of a first modification of the lens array;

FIG. 8 is a front view showing a part of a second modification of thelens array;

FIG. 9 generally shows an essential part of a fourth embodiment of thesynchronizing signal generating system according to the presentinvention applied to the document reading apparatus;

FIG. 10 shows relative positions of the lens array and the grating inthe fourth embodiment;

FIGS. 11A and 11B are diagrams for explaining the converging of light bythe lens array depending on the arrangement thereof;

FIG. 12 generally shows an essential part of a fifth embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus;

FIG. 13 shows relative positions of the lens array and the grating inthe fifth embodiment;

FIG. 14 generally shows an essential part of a sixth embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus;

FIGS 15A through 15C respectively show an essential part of the lensarray for explaining the convergence of light;

FIG. 16 generally shows an essential part of a seventh embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus or document readingapparatus;

FIG. 17 shows an essential part of an eighth embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus or document readingapparatus;

FIG 18 shows an essential part of the lens array for explaining theconvergence of light at a chamfered portion of the lens;

FIG. 19 generally shows an essential part of a ninth embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus;

FIG. 20 shows the linearity characteristic of the scanning position ofthe synchronizing beam transmitted through the fθ-lens with respect tothe synchronizing beam angle θ;

FIG. 21 is a diagram for explaining the moire which is introduced whendots are recorded on the recording medium;

FIG. 22 is a diagram for explaining ranges in which convex lenses arearranged in the ninth embodiment;

FIG. 23 is a diagram for explaining the non-existence of moire when thesynchronizing beam is transmitted through the grating in the ninthembodiment;

FIG. 24 is a system block diagram showing an embodiment of a phaselocked loop circuit shown in FIG. 19;

FIGS. 25(A) and 25(B) are time charts for explaining the operation ofthe phase locked loop circuit shown in FIG. 24;

FIG. 26 shows an essential part of a first modification of the ninthembodiment;

FIG. 27 shows an essential part of a second modification of the ninthembodiment;

FIG. 28 is a system block diagram showing an essential part of anelectrical system of a tenth embodiment of the synchronizing signalgenerating system according to the present invention;

FIGS. 29(A) and 29(B) are timing charts for explaining the operation ofthe electrical system shown in FIG. 28;

FIG. 30 is a block diagram showing an embodiment of a correcting circuitfor producing a corrected synchronizing clock signal from a pseudosynchronizing clock signal generated in the tenth embodiment;

FIGS. 31(A) and 31(B) are time charts for explaining a change in acontrol voltage of a voltage controlled oscillator within the phaselocked loop circuit caused by a change in a duty cycle of thesynchronizing clock signal;

FIG. 32 is a system block diagram showing an essential part of anelectrical system of an eleventh embodiment of the synchronizing signalgenerating system according to the present invention;

FIGS. 33(A) and 33(B) are timing charts for explaining the operation ofthe eleventh embodiment;

FIGS. 34(A) and 34(B) show signal waveforms for explaining the pull-intime of the PLL circuit;

FIGS. 35(A) and 35(B) and FIGS. 36(A) and 36(B) respectively are timingcharts for explaining the relationship between the synchronizing clocksignal and the pseudo synchronizing clock signal;

FIG. 37 is a system block diagram showing an essential part of anelectrical system of a twelfth embodiment of the synchronizing signalgenerating system according to the present invention;

FIGS. 38A) through 38(H) are timing charts for explaining the operationof the electrical system shown in FIG. 37;

FIG. 39 shows the synchronizing clock signal for explaining a change inthe duration of the non-image region due to an error in an angularseparation of two mutually adjacent mirror surfaces of the polygonalmirror;

FIGS. 40(A) and 40(B) and FIGS. 41(A) and 41(B) are timing charts forexplaining the phase shift between the synchronizing clock signal andthe pseudo synchronizing clock signal;

FIGS. 42A through 42D are timing charts for explaining the operatingprinciple of a thirteenth embodiment of the synchronizing signalgenerating system;

FIG. 43 is a system block diagram showing an essential part of anelectrical system of the thirteenth embodiment of the synchronizingsignal generating system according to the present invention;

FIG. 44 shows a detector of the electrical system shown in FIG. 43together with the polygonal mirror;

FIG. 45 is a system block diagram showing an embodiment of a timingcircuit of the electrical system shown in FIG. 43;

FIGS. 46(A) through 46(C) are timing charts for explaining the operationof the timing circuit shown in FIG. 45;

FIGS. 47(A) through 47(C) are timing charts for explaining the operationof the thirteenth embodiment;

FIG. 48 shows relative positions of a mirror array and the grating in afourteenth embodiment of the synchronizing signal generating systemaccording to the present invention; and

FIG. 49 generally shows an essential part of a fifteenth embodiment ofthe synchronizing signal generating system according to the presentinvention applied to the recording apparatus or document readingapparatus.

DETAILED DESCRIPTION

FIG. 3 shows an essential part of a first embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus. In FIG. 3, theillustration and description of those parts of the recording apparatuswhich are essentially the same as those of the recording apparatus shownin FIG. 1 will be omitted.

In FIG. 3, 0 denotes a point of deflection of the synchronizing beam ona mirror surface of the polygonal mirror (not shown). An optical pathfrom the point 0 to a grating 30 is identical to that of FIG. 1. Thegrating 10 shown in FIG. 2 may be used as the grating 30. For example,the bright and dark portions of the grating 30 are provided with a pitchwhich is an integral multiple of the recording density.

A lens array 32 comprises n lenses 32₁ through 32_(n) for independentlyconverging the synchronizing beam transmitted through the grating 30.The lenses 32₁ through 32_(n) are arranged side by side along a scanningdirection of the synchronizing beam for the full width of the scan. Thelens array 32 constitutes a converging optical system. A light receivingsystem 29A located on the rear of the lens array 32 comprises n lightreceiving elements 29A₁ through 29A_(n) which are also arranged alongthe scanning direction. These light receiving elements 29A₁ through29A_(n) are arranged in correspondence with the lenses 32₁ through32_(n) of the lens array 32.

The lens array 32 may be made of glass lenses or injection molded froman optical plastic. The lens array 32 may also be constituted byholographic lenses. When plastic lenses or holographic lenses are usedfor the lens array 32, the cost of the converging optical system can bekept down to a very low cost.

When the synchronizing beam transmitted through the grating 30 reachesthe lens of the lens array 32, this lens converges and directs thesynchronizing beam to a corresponding light receiving element of thelight receiving system 29A. In other words, the synchronizing beamreceived through the grating 30 is divided by the lens array 32 and thedivided beams are received by the corresponding light receiving elements29A₁ through 29A_(n) of the light receiving system 29A.

Outputs of the light receiving elements 29A₁ through 29A_(n) areamplified in corresponding amplifiers 34₁ through 34_(n) of an amplifierpart 34A and are added in an adder 36. An output pulse signal of theadder 36 is dependent on the arrangement of the bright and dark portionsof the grating 30, and this output pulse signal is supplied to theshaping circuit (not shown). The output of the shaping circuit isapplied to the laser diode driver (not shown) as a synchronizing clocksignal so as to control the recording timing (that is, scan timing) andan image clock signal is produced based on this synchronizing clocksignal as in the case of the conventional recording apparatus shown inFIG. 1 described before.

FIG. 4 shows an essential part of a second embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus. In FIG. 4, those partswhich are essentially the same as those corresponding parts in FIG. 3are designated by the same reference numerals, and a description thereofwill be omitted.

In FIG. 4, a light receiving system 29B comprises optical fibers 29B₁through 29B_(n), a lens 29B_(a) and a light receiving element 29B_(b).Receiving ends of these optical fibers 29B₁ through 29B_(n) are arrangedalong the scanning direction in correspondence with the n lenses 32₁through 32_(n) of the lens array 32. On the other hand, emitting ends ofthe optical fibers 29B_(l) through 29B_(n) are bundled and confront thelens 29B_(a). Hence, the lights emitted from the emitting ends of theoptical fibers 29B₁ through 29B_(n) are converged by the lens 29B_(a)onto the light receiving element 29B_(b). An output of the lightreceiving element 29B_(b) is amplified by an amplifier 34B and issupplied to the shaping circuit (not shown).

According to the second embodiment, the number of light receivingelements and amplifiers may be reduced compared to the first embodiment.

In the first and second embodiments, the laser diodes are used as thelight source. However, it is of course possible to use a gas laser asthe light source. But the gas laser cannot carry out a directmodulation, and in this case, it is necessary to use an externalmodulator such as an acoustic optical modulator (AOM). The generatedsynchronizing clock signal is applied to an AOM driver which drives theAOM.

FIG. 5 shows an essential part of a third embodiment of thesynchronizing signal generating system according to the presentinvention applied to a document reading apparatus. In FIG. 5, thoseparts which are essentially the same as those corresponding parts inFIG. 4 are designated by the same reference numerals, and a descriptionthereof will be omitted. In this third embodiment, a single gas lasersource 21 is used as the light source.

In FIG. 5, a laser beam from the gas laser source 21 is divided into alaser beam for reading (hereinafter referred to as a reading beam) and asynchronizing beam by a semitransparent mirror 22A. The reading beam isreflected by the semitransparent mirror 22A while the synchronizing beamis transmitted through the semitransparent mirror 22A. The reading beamis deflected by the polygonal mirror 23 and scans a document 50 throughthe fθ-lens 24. The reading beam reflected from the document 50 isreceived by a bundle of optical fibers 51. The reading beam received bythe optical fibers 51 is converted into an image signal by aphotoelectric conversion carried out in a light receiving element 52.

On the other hand, the synchronizing beam is reflected by a mirror 22Band is deflected by the polygonal mirror 23. The deflected synchronizingbeam passes through the fθ-lens 24 and is reflected by the mirrors 26and 27 so as to scan the grating 30. The synchronizing beam transmittedthrough the grating 30 is received by the light receiving system 29Bwhere it is subjected to the photoelectric conversion. The pulse signaloutputted from the light receiving system 29B is amplified in theamplifier 34B and is applied to a reading circuit 53 as thesynchronizing clock signal. The reading circuit 53 is also supplied withthe image signal from the light receiving element 52, and the readingcircuit 53 can detect the scanning position of the reading beam on thedocument 50 by counting pulses of the image clock signal which isgenerated based on the synchronizing clock signal. The image signalreceived by the reading circuit 53 is supplied to an image reproducingcircuit 54 which reproduces the image on the document 50 which isscanned.

In the first through third embodiments described heretofore, the lenses32₁ through 32_(n) of the lens array 32 have the same circular shape andthe peripheral portions of two mutually adjacent lenses touch each otheras shown in FIG. 6A. When the lens array 32 is correctly positioned, thesynchronizing beam scans along a scanning line SL indicated by a solidline in FIG. 6A. This scanning line SL passes centers of each of thelenses 32₁ through 32_(n). But when there is a positioning error of thelens array 32, the synchronizing beam scans along a scanning line SLeindicated by a phantom line in FIG. 6A, for example. As shown, thescanning line SLe is deviated from the scanning line SL, that is,deviated from the correct scanning position, and does not pass thecenters of the lenses 32₁ through 32_(n). When the synchronizing beamscans along the scanning line SLe, the synchronizing beam does not reachthe light receiving system at joint portions between the two mutuallyadjacent lenses of the lens array 32 because of the gap formed at thejoint portion. An example of the gap formed at the joint portion isindicated by J in FIG. 6A. As a result, a signal dropout occurs in thesynchronizing clock signal at the joint portions as shown in FIG. 6B.

Next, a description will be given on modifications of the lens array 32applicable to each of the first through third embodiments foreliminating the signal dropout in the synchronizing clock signal.

FIGS. 7A and 7B show a part of a first modification of the lens array32. A lens array 32A comprises cylindrical lenses 32Aa which arearranged side by side in a line. Each cylindrical lens 32Aa correspondsto a central rectangular portion of a circular lens indicated by aphantom line in FIG. 7A. Accordingly even when the synchronizing beamscans along a scanning line deviated from the correct scanning position,no signal dropout will occur in the synchronizing clock signal becausethere is no gap at the joint portion between two mutually adjacentcylindrical lenses 32Aa.

FIG. 8 shows a part of a second modification of the lens array 32. Alens array 32B comprises lenses 32Ba which are arranged side by side ina line. Each lens 32Ba corresponds to a central portion of a circularlens indicated by a phantom line. The gap formed at the joint portionbetween two mutually adjacent lenses 32Ba is extremely small compared tothat of the lens array 32 shown in FIG. 6A. Hence, even when thesynchronizing beam scans along a scanning line deviated from the correctscanning position, no signal dropout will occur in the synchronizingclock signal provided that the scanning line falls within a range R whenthe positioning error of the lens array 32B exists.

The lens arrays 32A and 32B may be made of glass lenses or injectionmolded from an optical plastic. The lens arrays 32A and 32B may also beconstituted by holographic lenses. When plastic lenses or holographiclenses are used for the lens arrays 32A and 32B, the cost of theconverging optical system can be kept down to a very low.

According to the first through third embodiments, the precision requiredof the lenses constituting the lens array is not extremely high as inthe case of the single concave mirror of the mirror array used in theconventional system, because the lens array as a whole receives thesynchronizing beam transmitted through the grating and the lens arraysupplies this synchronizing beam in division to the light receivingsystem. Thus, the lens array can be produced with ease at a low cost. Inaddition, the positioning and mounting of the lens array is easy becausethe lens array is located between the grating and the light receivingsystem. A distance between the grating and the light receiving systemcan be set small because the grating and the lens array can be providedclose together and the focal distance of the lenses constituting thelens array can be set short owing to the fact that the synchronizingbeam transmitted through the grating is received in division by theselenses. As a result, the recording apparatus or document readingapparatus can be made compact. Furthermore, when one of themodifications of the lens array is used, it is possible to compensatefor the positioning error of the lens array.

Next, a description will be given on a fourth embodiment of thesynchronizing signal generating system according to the presentinvention applied to the document reading apparatus, by referring toFIG. 9. In FIG. 9, those parts which are the same as those correspondingparts in FIG. 5 are designated by the same reference numerals, and adescription thereof will be omitted. The present embodiment uses thelens array 32A shown in FIGS. 7A and 7B described before, and there is apredetermined relationship in relative positions of the grating 30 andthe lens array 32A.

When the lens array 32A is used, a scattering of light occurs at ajunction portion between two mutually adjacent cylindrical lenses of thelens array 32A, and there inevitably is a decrease in the quantity oflight reaching the corresponding light receiving element from thejunction portion. As a result, the output of the light receiving elementdeviates and the synchronizing clock signal becomes unstable, therebymaking it impossible to carry out a highly accurate synchronousdetection at the junction portion. In order to ensure the generation ofa highly accurate synchronizing clock signal, it is necessary to providea compensation circuit to compensate for the output deviation of thelight receiving element caused by the scattering of the reflected lightat the junction portion, but the use of such a compensation circuitmakes the laser scanner complex. Similar problems occur at the boundaryportion between two mutually adjacent lenses of the lens array 32 and atthe junction portion between two mutually adjacent lenses of the lensarray 32B. Such problems also occur at a boundary portion between twomutually adjacent concave mirrors constituting a mirror array which maybe used in place of the lens array.

But according to the fourth embodiment, the relative positions of thegrating 30 and the lens array 32A are set as shown in FIG. 10. In FIG.10, Pg denotes a pitch of the bright and dark portions of the grating10, and P1 denotes a pitch of the cylindrical lenses 32Aa of the lensarray 32A. A ratio Pl/Pg is set to an integer so that the dark portionof the grating 30 confronts the junction portion between two mutuallyadjacent cylindrical lenses 32Aa of the lens array 32A. For example,Pl/Pg=2 in the present embodiment.

Therefore, the synchronizing beam transmitted through the grating 30will not reach the junction portion between the two mutually adjacentcylindrical lenses 32Aa of the lens array 32A, and the scattering oflight will not occur at the junction portion. It is thus possible togenerate a highly accurate synchronizing clock signal even at thejunction portion.

In the fourth embodiment, a flat side of the lens array 32A faces thegrating 30 as shown in FIG. 10. But as shown in FIG. 11A, the flat sideof the lens array 32A does not contribute to the converging of lightwhen the flat side of the lens array 32A is in close contact with thegrating 30. In other words, only the curved side of the lens array 32Acontributes to the converging of light, and the spherical aberration islarge. As a result, the light does not sharply converge, and in thiscase, it is necessary to use a light receiving element having arelatively large light receiving area. However, the operating speed ofsuch a light receiving element having the relatively large lightreceiving area is slow because of its poor frequency characteristic.From the point of view of enabling a high speed operation of therecording apparatus or document reading apparatus, it is preferable touse a light receiving element which has a small light receiving area andis operable at a high speed.

Similar problems occur when the light converged by the lens array 32A isfirst received by the receiving ends of the optical fibers 29B₁ through29B_(n). That is, when the lens array 32A does not sharply converge thelight at the receiving ends of the optical fibers 29B₁ through 29B_(n),the information based on which the synchronizing clock signal isgenerated is not accurately transmitted to the light receiving element29B_(b). In this case, it is difficult to generate a highly accuratesynchronizing clock signal.

On the other hand, if the synchronizing beam transmitted through thegrating 30 is first received by the curved side of the lens array 32A,both the curved side and the flat side of the lens array 32 contributeto the converging of light as shown in FIG. 11B. Hence, it can be seenthat it is more preferable to position the lens array 32A so that thecurved side thereof faces the grating 30.

FIG. 12 shows an essential part of a sixth embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus. In FIG. 12, those partswhich are the same as those corresponding parts in FIG. 3 are designatedby the same reference numerals, and a description thereof will beomitted.

In FIG. 12, a recording beam is emitted from a laser diode 44 and asynchronizing beam is emitted from a laser diode 45. The recording andsynchronizing beams are collimated by respective collimator lenses 46and 47 and reach a deflection beam splitter 48. The recording andsynchronizing beams from the deflection beam splitter 48 are directed tothe polygonal mirror 23 by a cylindrical lens 49 which is provided forbeam shaping. The recording and synchronizing beams land atapproximately the same positions on the mirror surface of the polygonalmirror 23 but with slightly different incident angles. Therefore, therecording and synchronizing beams are deflected along slightly differentdirections. Projections of the deflected recording and synchronizingbeams on a plane perpendicular to a rotary axis of the polygonal mirror23 intersect.

The recording beam indicated by a solid line reaches a recording medium58 such as a photosensitive drum or sheet by way of the fθ-lens 24 and amirror 28. Hence, the recording beam scans the recording medium 58 asthe polygonal mirror 23 rotates. For example, the recording beam scansin a direction B and the recording medium 58 is fed in a direction Cevery time one scan is completed. On the other hand, the synchronizingbeam indicated by a phantom line is directed to the grating 30 afterbeing transmitted through the fθ-lens 24. The synchronizing beam scansthe grating 30 as the polygonal mirror 23 rotates.

The output pulse signal of the adder 36 is dependent on the arrangementof the bright and dark portions of the grating 30, and this output pulsesignal is supplied to a shaping circuit 38. An output synchronizingclock signal of the shaping circuit 38 is supplied to a phase lockedloop circuit (clock control circuit) 39 which outputs the image clocksignal. This image clock signal is applied to a laser diode driver 40 soas to control the recording timing (that is, scan timing). The laserdiode driver 40 drives the laser diode 44 in synchronism with theincoming image clock signal depending on data entered from aninformation source 41. Since the synchronizing clock signal is generatedbased on the synchronizing beam, the driving timing of the laser diode44 is automatically adjusted even when the rotational speed of thepolygonal mirror 23 becomes unstable during the recording operation.Therefore, the recording operation is carried out with an appropriatescan timing. A laser diode driver 42 drives the laser diode 45.

In FIG. 12, the relative positions of the grating 30 and the lens array32A are set as shown in FIG. 13 so that the curved side of the lensarray 32A faces the grating 30. Although the grating 30 and the lensarray 32A are shown in FIG. 12 with a gap formed therebetween, it ismore desirable to provide the grating 30 and the lens array 32A closetogether as shown in FIG. 13.

In FIG. 13, the pitch Pl of the lens array 32A is selected to slightlygreater than an integral multiple of the pitch Pg of the grating 30.FIG. 13 shows the case where the pitch Pl is slightly greater than threetimes the pitch Pg. As a result, the light which would normally reachthe joint portion between the two mutually adjacent lenses 32Aa of thelens array 32A as indicated by a phantom line is positively blocked bythe grating 30 as indicated by a one-dot chain line. Hence, thescattering of light is prevented from occurring at the boundary portionbetween the two mutually adjacent lenses 32Aa of the lens array 32A, andthe stability of the synchronizing clock signal is ensured. Furthermore,since the curved side of the lens array 32A faces the grating 30, thelight is sharply converged on the corresponding light receiving elements29A₁ through 29A_(n) of the light receiving system 29A. Hence, ahigh-speed light receiving element having a small light receivingsurface may be used for the light receiving elements 29A₁ through29A_(n).

FIG. 14 shows an essential part of a sixth embodiment of thesynchronizing signal generating system according to the presentinvention applied to the recording apparatus. In FIG. 14, those partswhich are the same as those corresponding parts in FIGS. 4 and 12 aredesignated by the same reference numerals, and a description thereofwill be omitted.

According to the sixth embodiment, since the curved side of the lensarray 32A faces the grating 30, the light is sharply converged on thecorresponding receiving ends of the optical fibers 29B₁ through 29B_(n)of the light receiving system 29B. Thus, the information based on whichthe synchronizing clock signal is generated is accurately transmitted tothe light receiving element 29B_(b), and it is possible to generate ahighly accurate synchronizing clock signal.

The fifth and sixth embodiments may also be applied to the case where amirror array comprising mirrors are used in place of the lens array 32A,as will be described later in the specification.

According to the embodiments described heretofore, the optical axes ofthe lenses constituting the lens array are parallel to each other.However, in the case of the lens array 32, for example, thesynchronizing beam SB is received approximately perpendicularly to theflat side of the lens array 32 at the central portion of the lens array32 as shown in FIG. 15A, while the synchronizing beam SB is receivedwith an angle at the side portions of the lens array 32 as shown in FIG.15B. In other words, the synchronizing beam SB is approximately parallelto the optical axis of a lens 32_(cp) located at the central portion ofthe lens array 32, but the synchronizing beam SB is not parallel to theoptical axis of a lens 32_(sp) located at the side portions of the lensarray 32. As a result, the synchronizing beam SB converges sharply onlyat the central portion of the lens array 32 and not at the side portionsof the lens array 32. This means that the light receiving elements whichreceive the converged light must have a large light receiving area, butthe operation speed of such a light receiving element is slow because ofits poor frequency characteristic.

In addition, when the focal distance of the lenses constituting the lensarray 32, for example, is set short so as to make the recordingapparatus or document reading apparatus compact, the sphericalaberration becomes large and it becomes necessary to use a lightreceiving element having an extremely large light receiving area, as maybe seen from FIG. 15C. FIG. 15C shows a lens 32_(sfd) of the lens array32 having a short focal distance.

Similar problems occur when the converged light from the lens array 32or the like is received by the light receiving system 28B. In this case,the lens array 32 does not sharply converge the light at the receivingends of the optical fibers 29B₁ through 29B_(n), and the informationbased on which the synchronizing clock signal is generated is notaccurately transmitted to the light receiving element 29B_(b). Hence, itis difficult to generate a highly accurate synchronizing clock signal,as described before.

The above described problems caused by the non-uniform convergence ofthe light by the lens array depending on the scanning position of thesynchronizing beam also occurs when a mirror array is used in place ofthe lens array as the converging optical system.

Next, a description will be given on seventh and eighth embodiments ofthe synchronizing signal generating system applied to the recordingapparatus or document reading apparatus, in which these problems causedby the non-uniform convergence of the light by the converging opticalsystem are eliminated.

FIG. 16 shows an essential part of the seventh embodiment. In FIG. 16,those parts which are the same as those corresponding parts in FIG. 12are designated by the same reference numerals, and a description thereofwill be omitted. In the present embodiment, a converging optical system32C comprises lenses 32C₁ through 32C_(n) which are arranged along acurve as shown, so that the synchronizing beam transmitted through thegrating 30 is approximately parallel to the optical axes of the lenses32C₁ through 32C_(n). In other words, the synchronizing beam passing thecenters of the lenses 32C₁ through 32C_(n) coincide with the opticalaxes thereof. As a result, it is possible to satisfactorily converge thesynchronizing beam, and the converged beam spot caused by the aberrationis small. For this reason, a light receiving element having small lightreceiving area and operable at a high speed may be used for the lightreceiving elements 29A₁ through 29A_(n). Therefore, the seventhembodiment can be applied to a high speed recording apparatus ordocument reading apparatus.

FIG. 17 shows an essential part of the eighth embodiment. In FIG. 17,those parts which are the same as those corresponding parts in FIG. 12are designated by the same reference numerals, and a description thereofwill be omitted. In the present embodiment, a converging optical system32D comprises lenses 32D₁ through 32D_(n) which are arranged generallyalong a line as shown, so that the synchronizing beam transmittedthrough the grating 30 is approximately parallel to the optical axes ofthe lenses 32D₁ through 32D_(n). In other words, the synchronizing beampassing the centers of the lenses 32D₁ through 32D_(n) coincide with theoptical axes thereof. According to the present embodiment, it ispossible to obtain the same effects as those obtainable in the seventhembodiment.

It is possible to reduce the optical path between the fθ-lens 24 and thegrating 30 (and the converging optical system) by using a wide anglelens for the fθ-lens 24. But in the case of the lens array 32A describedbefore, for example, the convergence of light at the peripheral portionof each lens of the lens array 32A is poor especially because the edgeportion of the lens is usually chamfered. The edge portion of the lensis chamfered to prevent the edge portion from chipping. Consequently, atthe peripheral portion of the lens, the synchronizing beam may notconverge correctly as indicated by a phantom line in FIG. 18. When thesynchronizing beam is kicked at the chamfered edge portion of the lensand does not reach the corresponding light receiving element, a correctsynchronizing clock signal cannot be obtained.

But in the seventh and eighth embodiments, the optical axes of thelenses 32C₁ through 32C_(n) (or 32D₁ through 32D_(n)) of the convergingoptical system 32C (or 32D) coincide with the synchronizing beam whichpasses the centers of these lenses 32C₁ through 32C_(n) (or 32D₁ through32D_(n)) For this reason, the above described problem caused by thechamfered edge portion of the lenses will not occur even when the wideangle lens is used for the fθ-lens 24.

Although an illustration thereof will be omitted, it is evident that theconverging optical systems 32C and 32D can be applied to the recordingapparatus or document reading apparatus which uses the light receivingsystem 29B. In addition, the seventh and eighth embodiments arebasically applicable to the case where the mirror array comprising theconcave mirrors is used in place of the lens array, as will be describedlater in the specification.

According to the embodiments described heretofore which use theplurality of light receiving elements in the light receiving system, thenumber of light receiving elements used is large. Consequently, therecording apparatus or document reading apparatus using such a lightreceiving system becomes expensive.

Next, a description will be given on a ninth embodiment of thesynchronizing signal generating system according to the presentinvention, in which the number of light receiving elements can beeffectively reduced.

FIG. 19 shows an essential part of the ninth embodiment. In FIG. 19,those parts which are the same as those corresponding parts in FIG. 12are designated by the same reference numerals, and a description thereofwill be omitted. The present embodiment comprises a converging opticalsystem 32E and a light receiving system 29E. The converging opticalsystem 32E comprises convex lenses 32E₁ through 32E₃ and the lightreceiving system 29E comprises light receiving elements 29E₁ through29E₃ provided in correspondence with the convex lenses 32E₁ through32E₃.

The convex lenses 32E₁ through 32E₃ are arranged at predeterminedintervals along the scanning direction of the synchronizing beam. Thelight receiving elements 29E₁ through 29E₃ are arranged at suchpositions that the light transmitted through the convex lenses 32E₁through 32E₃ are converged on respective light receiving areas of thelight receiving elements 32E₁ through 32E₃. In this case, thesynchronizing clock signal Ps is obtained intermittently from theshaping circuit 38.

The linearity characteristic of the scanning position of thesynchronizing beam transmitted through the fθ-lens 24 with respect tothe synchronizing beam angle θ is shown in FIG. 20. For this reason,when the dots are recorded on the recording medium 58 with a constantperiod by the recording beam which is also transmitted through thefθ-lens 24, a moire occurs as shown in FIG. 21. For this reason, theconvex lenses 32E₁ through 32E₃ are arranged within such ranges so as toinclude inflection points IP₁ through IP₃ of the linearitycharacteristic of the fθ-lens 24.

Since the inflection points IP₁, IP₂ and IP₃ respectively occur at thesynchronizing beam angles θ of +α, 0 and -α, the convex lenses 32E₁,32E₂ and 32E₃ are arranged as shown in FIG. 22. That is, the convexlenses 32E₁, 32E₂ and 32E₃ are arranged within respective ranges R1, R2and R3 centered to converge at the synchronizing beam angles θ of +α, 0and -α, respectively. According to this arrangement of the convex lenses32E₁, 32E₂ and 32E₃, the positional error (moire) in the recordingposition of the dots caused by the linearity characteristic of thefθ-lens 24 can be suppressed to a small value ΔL. As a result, the moiredoes not occur when the synchronizing beam transmitted through thegrating 30 reaches the positions of the convex lenses 32E₁, 32E₂ and32E₃, that is, the ranges R1, R2 and R3, as may be seen from FIG. 23. InFIG. 23, IM denotes an imaginary plane on which the lenses 32E₁, 32E₂and 32E₃ are provided, and this imaginary plane IM is parallel to thegrating 30. For convenience sake, the imaginary plane IM is shownseparated from the grating 30, but this imaginary plane IM is actuallylocated immediately on the rear of the grating 30.

FIG. 24 shows an embodiment of the PLL circuit 39, and FIGS. 25(A) and25(B) are time charts for explaining the operation of the PLL circuit39. The PLL circuit 39 comprises a phase comparator (phase detector) 61,a frequency divider 62, a lowpass filter 63, an analog switch 64, acapacitor 65, and a voltage controlled oscillator (VCO) 66.

The synchronizing clock signal Ps is supplied intermittently to thephase comparator 61 because the convex lenses 32E₁, 32E₂ and 32E₃ arearranged within the respective ranges R1, R2 and R3 shown in FIG. 22 soas to converge the synchronizing beam. No synchronizing clock signal Psis supplied to the phase comparator 61 within ranges R10, R20 and R30.The synchronizing clock signal Ps is shown in FIG. 25(A). The phasecomparator 61 detects a phase error between the output synchronizingclock signal Ps of the shaping circuit 38 and an output signal of thefrequency divider 62. An output signal of the phase comparator 61dependent on a phase error between the two compared signals is passedthrough the lowpass filter 63, the analog switch 64 and the capacitor 65and applied to the VCO 66 to control an oscillation frequency thereof.An output signal of the VCO 66 is supplied to the frequency divider 62and the VCO 66 oscillates in synchronism with the output synchronizingclock signal Ps of the shaping circuit 38. The output signal of the VCO66 is outputted as the image clock signal.

The analog switch 64 is turned ON and OFF responsive to a signal T shownin FIG. 25(B). Immediately before the time periods corresponding to theranges R10, R20 and R30 in which no synchronizing clock signal Ps isreceived from the shaping circuit 38, the analog switch 64 is alreadyturned OFF by the signal T. For example, the analog switch 64 is turnedOFF by the signal T immediately before a time period t2, and thecapacitor 65 holds the input voltage to the VCO 66 before thesynchronizing clock signal Ps ceases. Accordingly, the oscillationfrequency of the VCO 66 is suppressed. In addition, the synchronizingclock signal Ps is received during the time periods corresponding to theranges R1, R2 and R3. For example, during a time period t1, thesynchronizing clock signal Ps from the shaping circuit 38 is supplied tothe phase comparator 61, and the analog switch 64 is turned ON to applythe output signal of the lowpass filter 63 to the VCO 66 through thecapacitor 65 when the output signal of the lowpass filter 63 reaches avoltage near the voltage held by the capacitor 65.

Therefore, the image clock signal is controlled by the synchronizingclock signal which is derived from the synchronizing beam transmittedthrough the grating 30 within the ranges R1, R2 and R3 including theinflection points IP₁, IP₂ and IP₃ of the linearity characteristic ofthe fθ-lens 24. On the other hand, within the ranges R10, R20 and R30,it is possible to suppress the error in the linearity characteristic ofthe fθ-lens 24 to ΔL by holding the input voltage of the VCO 66 duringthe times corresponding to the ranges R10, R20 and R30. In other words,the positional error (moire) in the recording position of the dots canbe corrected.

In the present embodiment, it is assumed that the mirror 28 reflects therecording beam onto the recording medium 58 as in the recordingapparatus shown in FIG. 12. However, as a first modification of theninth embodiment, it is possible to employ a reverse arrangement inwhich the synchronizing beam is reflected by a mirror 28A toward thegrating 30 as shown in FIG. 26. In FIG. 26, those parts which are thesame as those corresponding parts in FIG. 19 are designated by the samereference numerals, and a description thereof will be omitted.

FIG. 27 shows an essential part of a second modification of the ninthembodiment. In FIG. 27, those parts which are the same as thosecorresponding parts in FIG. 19 are designated by the same referencenumerals, and a description thereof will be omitted. Plates 68₁, 68₂ and68₃ respectively block the grating 30 within the ranges R10, R20 andR30. Hence, it is possible to prevent a noise from mixing into thesynchronizing clock signal Ps due to a light component which may enterthe convex lenses 32E₁ through 32E₃ through the grating 30 within theranges R10, R20 and R30.

As a third modification of the ninth embodiment, it is possible to use agrating which has the bright and dark portions (or slits and non-slits)formed only within the ranges R1, R2 and R3.

Therefore, according to the ninth embodiment and the modificationsthereof, it is possible to obtain an accurate synchronizing clock signalby use of only a small number of lenses in the converging optical systemand a corresponding small number of light receiving elements in thelight receiving system.

As described before for the conventional apparatus using the mirrorarray, the duty cycle of the synchronizing clock signal also becomesunstable at the boundary portion between the two mutually adjacentlenses of the lens array such as those used in FIGS. 3, 4, 5, 9, 12 and14. In extreme cases, a signal dropout occurs at the boundary portion.Usually, the PLL circuit 39 is used to match the phase of the imageclock signal with that of the synchronizing clock signal, and the imageclock signal is used to enable and disable the recording operation.Similarly as in the case shown in FIG. 24, the PLL circuit 39 comprisesa phase comparator for comparing the phases of the synchronizing clocksignal from the shaping circuit and an output signal of a VCO which iscontrolled by an output control voltage of the phase comparator, and theoutput signal of the VCO is used as the image clock signal.

For this reason, the unstable duty cycle of the synchronizing clocksignal and the signal dropout in the synchronizing clock signal causethe PLL circuit to run from the locked state and cause a sudden changein the oscillation frequency of the VCO. In these cases, thesynchronizing clock signal becomes unstable and deteriorates the qualityof the recording made on the recording medium. In terms of thefrequency, the change in the duty cycle of the synchronizing clocksignal causes a voltage change in the output control voltage of thephase comparator, and this change in the control voltage causes afrequency change in the image clock signal. The change in the frequencyof the image clock signal appears as moire and the like on the recordingmedium and greatly deteriorates the quality of the recording.

Next, descriptions will be given on embodiments of the synchronizingsignal generating system which compensates for the signal dropout in thesynchronizing clock signal electrically.

FIG. 28 shows an essential part of an electrical system of a tenthembodiment of the synchronizing signal generating system according tothe present invention. The electrical system includes a pseudosynchronizing clock signal generating circuit. The pseudo synchronizingclock signal generating circuit comprises counters 70 and 71, flip-flops72 and 73, an inverter 74, and an oscillator 75.

The synchronizing clock signal Ps shown in FIG. 29(A) is outputted fromthe shaping circuit 38 such as that shown in FIG. 12 and is supplied toa clock terminal of the counter 70. The synchronizing clock signal Psincludes portions 2-1 through 2-3 where the duty cycle varies and aportion 2-3 where a signal dropout occurs. These portions 2-1 through2-4 each correspond to the joint between two mutually adjacent lenses ofthe lens array (for example, 32A) which constitutes the convergingoptical system. When the counter 70 counts a pulse P1 shown in FIG.29(A) which occurs immediately before an unstable pulse at the portion2-1 corresponding to the joint, the counter 70 supplies a signal to aclock terminal of the D flip-flop 72. A high-level signal is constantlyapplied to an input terminal D of the flip-flop 72. When the flip-flop72 receives the output signal of the counter 70, the flip-flop 72releases the counter 71 from the load state and puts the counter 71 intoan enable state by supplying a signal to a load terminal LD of thecounter 71. In addition, this output signal of the flip-flop 72 issupplied to a load terminal LD of the counter 70 through the inverter 74so as to put the counter 70 into a load state.

When the counter 71 assumes the enable state, the counter 71 starts tocount output pulses of the oscillator 75. The output pulses of theoscillator 75 are high-frequency pulses having a sufficient resolutionwith respect to the synchronizing clock signal Ps. Ideal set times T1and T2 are preset in the counter 71 depending on the ideal pulse form ofthe synchronizing clock signal Ps, and the counter 71 outputs a signals1 when the count reaches the set time T1 and outputs a signal s2 whenthe count reaches the set time T2. The signals s1 and s2 arerespectively supplied to terminals J and K of the JK flip-flop 73. Theflip-flop 73 generates a pseudo synchronizing clock pulse P0 shown inFIG. 29(B) having a pulse width (T2-T1). The pulse width (T2-T1) is setso as to coincide with the correct duty cycle To of the synchronizingclock signal Ps.

The output signal s2 of the counter 71 is also supplied to a clearterminal CL of the flip-flop 72. For this reason, the flip-flop 72returns the counter 70 to the enable state responsive to the signal s2.The counter 70 thus starts to count the synchronizing clock signal Psfrom a pulse P2 thereof. On the other hand, the counter 71 assumes theload state until the counter 70 counts a pulse P3 of the synchronizingclock signal Ps. Similarly as in the case described before, theflip-flop 73 generates a pseudo synchronizing clock pulse P01 having apulse width (T2-T1) when the counter 70 counts the pulse P3. Byrepeating such operations, the flip-flop 73 generates pseudosynchronizing clock pulses P0, P01, P02, . . . , P0n having identicalpulse width and phase as the pulses constituting the synchronizing clocksignal Ps, at the portions 2-1 through 2-4 where the duty cycle of thesynchronizing clock signal Ps changes or a signal dropout occurs in thesynchronizing clock signal Ps.

Therefore, the portions 2-1 through 2-4 of the synchronizing clocksignal Ps where the duty cycle is unstable or the signal dropout occurscan be replaced by pseudo synchronizing clock pulses P0, P01, . . . ,P0n of a pseudo synchronizing clock signal PPs shown in FIG. 29(B).Accordingly, it is possible to obtain a synchronizing clock signal Pshaving a stable duty cycle and no signal dropout.

The joint between two mutually adjacent lenses of the lens array whichconstitutes the converging optical system occurs periodically, and therelationship between the pulses of the synchronizing clock signal Ps andthe joints is determined by the mechanical relationship between thegrating and the converging optical system. Hence, it is easy todetermine how many pulses of the synchronizing clock signal Ps would bereceived between two successive joints. In other words, in order for thecounter 70 to count the pulses P1 and P3 shown in FIG. 29(A) as thepulses immediately before the respective joints in the convergingoptical system, it is simply necessary to predict from the abovedescribed relationship how many pulses of the synchronizing clock signalPs would be received from one joint to immediately before the nextjoint.

FIG. 30 shows an embodiment of a correcting circuit for correcting thesynchronizing clock signal Ps. The synchronizing clock signal Ps fromthe shaping circuit 38 and the pseudo synchronizing clock signal PPsfrom the pseudo synchronizing clock signal generating circuit shown inFIG. 28 are supplied to a selector 77. The selector 77 selectivelyoutputs the pseudo synchronizing clock signal PPs during the timeperiods corresponding to the junction in the converging optical systemand outputs the synchronizing clock signal Ps during other periods. As aresult, a corrected synchronizing clock signal Psc having a stable dutycycle and no signal dropout is obtained from the selector 77. Thiscorrected synchronizing clock signal Psc is supplied to the PLL circuit39.

According to the tenth embodiment, it is possible to effectively preventthe duty cycle of the synchronizing clock signal from becoming unstableand prevent a signal dropout in the synchronizing clock signal. Anaccurate synchronizing lock signal is obtainable even at the portionscorresponding to the joints in the converging optical system by use ofthe pseudo synchronizing clock pulses generated in the pseudosynchronizing clock signal generating circuit.

Next, a description will be given on an eleventh embodiment of thesynchronizing signal generating system according to the presentinvention in which the undesirable effects caused by unstable duty cycleof the synchronizing clock signal are eliminated.

As described before, the unstable duty cycle of the synchronizing clocksignal and the signal dropout in the synchronizing clock signal causethe PLL circuit to run from the locked state and cause a sudden changein the oscillation frequency of the VCO within the PLL circuit. In thesecases, the image clock signal becomes unstable and deteriorates thequality of the recording made on the recording medium. And, in terms ofthe frequency, a change ΔD in the duty cycle of the synchronizing clocksignal Ps shown in FIG. 31(A) causes a voltage change ΔVc in the outputcontrol voltage Vc of the phase comparator shown in FIG. 31(B) forcontrolling the VCO, and this change in the control voltage Vc causes afrequency change in the image clock signal which is outputted from theVCO. The change in the frequency of the image clock signal appears asmoire and the like on the recording medium and greatly deteriorates thequality of the recording.

FIG. 32 shows an essential part of an electrical system of the eleventhembodiment of the synchronizing signal generating system according tothe present invention. The electrical system includes a correctioncircuit. The correction circuit comprises a flip-flop 80, a counter 81,an oscillator 82 and an edge detector 83.

As shown in FIG. 33(A), the synchronizing clock signal Ps should have aconstant duty cycle To but the duty cycle inevitably changes at thejoint portions of the converging optical system. This synchronizingclock signal Ps including the inevitable change in the duty cycle issupplied to the flip-flop 80. The flip-flop 80 sets the counter 81 andthe edge detector 83 to a ready state at the rising edge of thesynchronizing clock signal Ps, by supplying an output signal of theflip-flop 80 to a load terminal LD of the counter 81 and to a clearterminal CL of the edge detector 83.

The oscillator 82 outputs a pulse signal having a sufficiently highfrequency with respect to the synchronizing clock signal Ps, that is, asufficiently high resolution with respect to the synchronizing clocksignal Ps. This output pulse signal of the oscillator 82 is supplied toa clock terminal of the counter 81. The counter 81 counts pulses of thepulse signal received from the oscillator 82 when set to the ready stateby the flip-flop 80, and produces a pulse signal Psc having apredetermined time width To, that is, the duty cycle To. This pulsesignal Psc is outputted as the corrected synchronizing clock signal onone hand, and is supplied to a clock terminal of the edge detector 83 onthe other.

The edge detector 83 clears the flip-flop 80 by supplying a signal to aclear terminal CL of the flip-flop 80 when a falling edge of the pulsesignal Psc is detected. When the flip-flop 80 is cleared, the counter 81is returned to the initial state, that is, the load state, and the edgedetector 83 is cleared. The flip-flop 80 is released from the clearedstate when the edge detector 83 is cleared and the correcting circuit isready to detect the next pulse of the synchronizing clock signal Ps. Asa result, a pulse of the corrected synchronizing clock signal Psc shownin FIG. 33(B) which has the constant duty cycle To is generated everytime the pulse of the synchronizing clock signal Ps is received. Thecorrected synchronizing clock signal Psc is supplied to the PLL circuit(not shown).

Since the duty cycle To of the corrected synchronizing clock signal Pscis constant, no change is generated in the control voltage Vc of the VCOwithin the PLL circuit. Accordingly, the frequency of the image clocksignal generated from the corrected synchronizing clock signal Psc isstable, thereby enabling the laser beam to scan with an accurate scantiming. Of course, the present embodiment cannot cope with the casewhere a signal dropout occurs in the synchronizing clock signal Ps, andit is assumed that some measure is taken to ensure that no signaldropout occurs and only the change in the duty cycle of thesynchronizing clock signal Ps may occur.

Next, description will be given on embodiments of the synchronizingsignal generating system according to the present invention in which apseudo synchronizing clock signal is generated to correct unstableportions of the synchronizing clock signal.

As shown in FIG. 34(A), the synchronizing clock signal Ps which issupplied to the PLL circuit is generated intermittently, that is,generated in an image region and not generated in a non-image region.The image region refers to a scan duration in which the recording beamscans the recording medium and the recording is made in the case of therecording apparatus, while the non-image region refers to a non-scanduration in which the recording beam makes no recording on the recordingmedium in the case of the recording apparatus.

In the non-image region, the VCO of the PLL circuit oscillates at thefree-running frequency f_(o) and the output of the VCO is obtained asthe output of the PLL circuit, that is, the image clock signal shown inFIG. 34(B). For this reason, even when the synchronizing clock signal Psis received subsequent to the non-image region, it takes a pull-in timet_(p) for the oscillation frequency of the VCO to stabilize so that theoutput of the VCO can be used as the image clock signal. Normally, thelength of the grating along the scanning direction of the synchronizingbeam is extended by a length corresponding to the pull-in time t_(p) tostart the recording after the oscillation frequency of the VCOstabilizes. However, the oscillation frequency of the VCO changes to atemperature change, and this oscillation frequency change causes achange in the pull-in time t_(p). Consequently, the PLL circuit may notbe locked when the image region is reached thereby making it impossibleto correctly synchronize each scan.

It is possible to conceive a method of sampling the holding the controlvoltage supplied to the VCO, but in this case, a discontinuity occursbetween the sampled value in the image region and the held value in thenon-image region because this method is also easily affected by thetemperature change. As a result, this discontinuity causes instabilityin the oscillation frequency of the VCO, and it is impossible tocorrectly synchronize each scan.

In a twelfth embodiment of the synchronizing signal generating systemaccording to the present invention, the pseudo synchronizing clocksignal is generated within the entire non-image region. FIGS. 35(A) and35(B) show the relationship between the synchronizing clock signal Psand the pseudo synchronizing clock signal PPs generated in the presentembodiment. FIGS. 36(A) and 36(B) respectively show encircled portionsof the synchronizing clock signal Ps and the pseudo synchronizing signalPPs on an enlarged scale. Ta denotes the scanning period, ts denotes theperiod of the synchronizing clock signal Ps, tps denotes the period ofthe pseudo synchronizing clock signal PPs, n1 denotes the number ofpulses of the synchronizing clock signal Ps in one scanning period Ta,n2 denotes the number of pulses of the pseudo synchronizing clock signalPPs in one scanning period Ta, and t_(o) denotes the duration of thenon-image region. A relation Ta=n×ts stands, where n=n1+n2.

FIG. 37 shows an essential part of an electrical system of the twelfthembodiment of the synchronizing signal generating system according tothe present invention, and FIGS. 38(A) through 38(H) are timing chartsfor explaining the operation of the electrical system shown in FIG. 37.The electrical system includes a correcting circuit for correcting thesynchronizing clock signal Ps by use of the pseudo synchronizing clocksignal PPs. The correcting circuit comprises counters 91 through 93,flip-flops 94 through 96, a timer 97, an oscillator 98, inverters 99₁through 99₄ and an OR circuit 100 which are connected as shown in FIG.37.

The synchronizing clock signal Ps shown in FIG. 38(A) is applied to theinverter 99₂ and an inverted synchronizing clock signal Ps shown in FIG.38(B) is applied to a clock terminal of the counter 91. The invertedsynchronizing clock signal Ps is again inverted in the inverter 99₁ andis applied to a clock terminal of the timer 97. The timer 97 outputs apulse signal S1 shown in FIG. 38(C) having a pulse width PW satisfyingts<PW<t_(o) -ts. The counter 91 loads a number (n1-2) of pulses to becounted responsive to a first rise r1 in the inverted synchronizingclock signal Ps, that is, responsive to the pulse signal S1. And, thecounter 91 starts to count the pulses responsive to a second rise r2 inthe inverted synchronizing clock signal Ps. When (n1-2) pulses arecounted in the counter 91, the counters 92 and 93 are set to readystates by an output signal S2 of the flip-flop 94 shown in FIG. 38(D).The counter 92 counts output pulses of the oscillator 98 having asufficiently high resolution and outputs a signal S3 shown in FIG. 38(E)for every ts/2. The flip-flop 95 frequency-divides the signal S3 by twoand outputs a pulse signal S4 shown in FIG. 38(F). The counter 93 countsan inverted signal S4 which is obtained through the inverter 99₄, andoutputs a signal S5 shown in FIG. 38(G) when n2 pulses are counted. Thesignal S5 is applied to a clock terminal of the flip-flop 96. Theflip-flop 96 outputs a signal S6 shown in FIG. 38(H) for clearing theflip-flop 94. When the flip-flop 94 is cleared, the counter 92 isdisabled, while the counter 93 and the flip-flop 96 are cleared. Theflip-flop 94 assumes the ready state with respect to the output of thecounter 91 when the flip-flop 96 is cleared. The timer 97 clears thecounted value of the counter 91 in the non-image region.

The OR circuit 100 is supplied with the synchronizing clock signal Psand the pseudo synchronizing clock signal PPs outputted from theflip-flop 95. Hence, a corrected synchronizing clock signal Psc isoutputted from the OR circuit 100 and is supplied to the PLL circuit.The corrected synchronizing clock signal Psc is made up of the pulseshaving the period ts in both the image region and the non-image region.Accordingly, the pull-in time for the PLL circuit to lock in is onlyrequired when the power source is turned ON, and once locked in, the PLLcircuit can generate a stable image clock signal from the correctedsynchronizing clock signal Psc for the entire image region. This meansthat the length of the grating need only correspond to the length of theimage region, and it is possible to reduce both the overall size andcost of the recording apparatus or document reading apparatus appliedwith the synchronizing signal generating system.

According to the present embodiment, the frequency of the pseudosynchronizing clock signal PPs is identical to the frequency of thesynchronizing clock signal Ps in the image region. In addition, thesynchronizing clock signal Ps and the pseudo synchronizing clock signalPPs are generated so that the frequencies thereof are an integralmultiple of the scanning frequency.

Next, a description will be given on a thirteenth embodiment of thepseudo synchronizing signal generating system according to the presentinvention. As indicated by t_(e1) and t_(e2) in FIG. 39, durations ofthe non-image region may become different due to an error in the angularseparation of two mutually adjacent mirror surfaces of the polygonalmirror. Ideally, the angular separation of two mutually adjacent mirrorsurfaces of the polygonal mirror is constant, but an error inevitablyoccurs in the production process. The difference between the durationste1 and te2 is usually within Tps/2, where Tps denotes the period of thesynchronizing clock signal Ps. Hence, when the pseudo synchronizingclock signal PPs is generated after a predetermined time from the end ofthe synchronizing clock signal Ps, the phase of the pseudo synchronizingclock signal PPs shown in FIG. 40(A) may be matched to the phase of thesynchronizing clock signal Ps shown in FIG. 40(B) at a time tA, but thephase of the pseudo synchronizing clock signal PPs shown in FIG. 41(A)may be inverted with respect to the phase of the synchronizing clocksignal Ps shown in FIG. 41(B) at a time tB. In FIG. 41(A), Δte denotes aphase shift in the synchronizing clock signal Ps caused by the error inthe angular separation between two mutually adjacent mirror surfaces ofthe polygonal mirror. In this case, the output of the VCO in the PLLcircuit becomes unstable, that is, the image clock signal becomesunstable. Furthermore, there is a danger in that the PLL circuit may runfrom the locked state due to the unstable output of the VCO.

In the thirteenth embodiment, the generating timing of the pseudosynchronizing clock signal PPs is varied appropriately so as to matchthe phase of the pseudo synchronizing clock signal PPs with the phase ofthe synchronizing clock signal Ps, to generate a stable image clocksignal. In other words, the pseudo synchronizing clock signal PPs isgenerated as shown in FIGS. 42B through 42D at the times tA0, tA1 andtA2 indicated in FIG. 42A which shows the synchronizing clock signal Ps.The time scales of FIGS. 42B through 42D are enlarged compared to thatof FIG. 42A. The durations t_(e1), t_(e2), t_(e3), . . . , t_(em) of thenon-image region can be obtained with ease, where m denotes the numberof mirror surfaces of the polygonal mirror.

When it is assumed that the duration t_(e1) with the smallest subscriptis the shortest of the durations t_(e1) through t_(em), the pseudosynchronizing clock signal PPs is generated as shown in FIG. 42B. Inother words, after the synchronizing clock signal Ps ends at a time tA0shown in FIG. 42A, the pseudo synchronizing clock signal PPs isgenerated after a time t_(e0) from the time tA0. In this case, the phaseof the pseudo synchronizing clock signal PPs matches the phase of thesynchronizing clock signal Ps at the time tB0 in FIG. 42B. The pseudosynchronizing clock signal PPs is generated after the synchronizingclock signal Ps ends at the time tA1 in FIG. 42C, with a timing Δte2delayed with respect to the generating timing of FIG. 42B. Hence, thephase of the pseudo synchronizing clock signal PPs matches the phase ofthe synchronizing clock signal Ps at the time tB1 in FIG. 42C. Thepseudo synchronizing clock signal PPs is generated after thesynchronizing clock signal Ps ends at the time tA2 in FIG. 42D, with atiming Δte3 delayed with respect to the generating timing of FIG. 42B.Thus, the phase of the pseudo synchronizing clock signal PPs matches thephase of the synchronizing clock signal Ps at the time tB2 in FIG. 42D.Therefore, the phase of pseudo synchronizing clock signal PPs matchesthe phase of the synchronizing clock signal Ps at the time tBM becauseafter the synchronizing clock signal Ps ends at a time tAM, the pseudosynchronizing clock signal PPs is generated with a timing ΔteM delayedwith respect to the generating timing of FIG. 42B, where ΔteM=t_(eM)-t_(e1) and M=2, 3, . . . , m.

FIG. 43 shows an essential part of an electrical system of thethirteenth embodiment of the synchronizing signal generating systemaccording to the present invention. The electrical system includes acorrecting circuit. The correcting circuit comprises a detector 110, atiming circuit 111, a pseudo synchronizing clock signal generatingcircuit 112 and an OR circuit 113.

The detector 110 detects the scanning mirror surface of the polygonalmirror. As shown in FIG. 44, the detector 110 comprises a light source110a and a light receiving element 110b. A mark 23A is provided on a topsurface of the polygonal mirror 23, and this mark 23A has a coefficientof reflection different from that at other portions of the top surface.The light emitted from the light source 110a is reflected at the topsurface of the polygonal mirror 23 and is received by the lightreceiving element 110b. Since the light emitted from the light source110a hits the mark 23A once every revolution of the polygonal mirror 23,it is possible to know from an output detection signal of the lightreceiving element 110b the timing of each image region (scan duration),that is, the scanning mirror surface of the polygonal mirror 23. Thisoutput detection signal of the light receiving element 110b is suppliedto the timing circuit 111. The timings (or delay timings) with which thepseudo synchronizing clock signal PPs is to be generated can bedetermined from the output detection signal of the detector 110. Thetimes t_(e1), t_(e2), . . . , t_(em) do not change, and the m timingscan be used repeatedly once determined.

FIG. 45 shows an embodiment of the timing circuit 111. The timingcircuit 111 comprises a counter 121 and a data selector 122. A clocksignal CLK shown in FIG. 46(C) is applied to a clock terminal of thecounter 121. This clock signal CLK occurs once within each image regionof the synchronizing clock signal Ps shown in FIG. 46(B). The outputdetection signal D of the detector 110 is applied to a clear terminal CLof the counter 121. Thus, the counter 121 counts pulses of the clocksignal CLK and is cleared responsive to the detection signal D. Thecounted value in the counter 121 is supplied to the data selector 122.The data selector 122 is supplied with the durations t_(e1) throught_(em) and selectively outputs one of the durations depending on thecounted value from the counter 121. The output of the data selector 122is supplied to the pseudo synchronizing clock signal generating circuit112 which generates the pseudo synchronizing clock signal PPs based onthe timing determined by the output of the data selector 122.

Returning now to the description of FIG. 43, the OR circuit 113 issupplied with the synchronizing clock signal Ps and the pseudosynchronizing clock signal PPs from the pseudo synchronizing clocksignal generating circuit 112, and outputs the corrected synchronizingclock signal Psc. FIGS. 47(A), 47(B) and 47(C) respectively show thesynchronizing clock signal Ps, the pseudo synchronizing clock signal PPsand the image clock signal I_(CL) outputted from the PLL circuitresponsive to the corrected synchronizing clock signal Psc from the ORcircuit 113. As may be seen from FIGS. 47(A) through 47(C), the imageclock signal I_(CL) may become unstable after one image region where thesynchronizing clock signal Ps ends, due to the discontinuity of thepulses supplied to the PLL circuit. However, the start of thesynchronizing clock signal in the next image region is in phase with thepseudo synchronizing clock signal PPs generated in the non-image regionimmediately preceding this next image region. Hence, the stability ofthe image clock signal I_(CL) is substantially maintained, even whenthere is an error in the angular separation between two mutuallyadjacent mirror surfaces of the polygonal mirror.

According to the present embodiment, the frequency of the pseudosynchronizing clock signal PPs is identical to the frequency of thesynchronizing clock signal Ps in the image region. In addition, thesynchronizing clock signal Ps and the pseudo synchronizing clock signalPPs are generated so that the phases thereof match each other.

The tenth through thirteenth embodiments are basically applicable to thecase where a mirror array comprising mirrors is used in place of thelens array, as will be described later in the specification.

Next, descriptions will be given with respect to applications of thefifth through eight embodiments to the recording apparatus or documentreading apparatus employing a mirror array in place of the lens array.

In a fourteenth embodiment of the synchronizing signal generating systemaccording to the present invention, the lens array 32A is replaced by amirror array comprising concave mirrors in the fifth and sixthembodiments shown in FIGS. 12 and 14 described before. FIG. 48 shows therelationship of the grating 30, the mirror array and the light receivingsystem. The remaining parts of the recording apparatus or documentreading apparatus may be the same as those of the embodiments describedheretofore, and an illustration and description thereof will be omitted.

In FIG. 48, the synchronizing beam transmitted through the grating 30 isreflected by the concave mirrors of the mirror array 200 and isconverged at a light receiving system 201. The light receiving system201 may comprise the light receiving elements as in the case of thefifth embodiment, or the optical fibers as in the case of the sixthembodiment. In FIG. 48, Pm denotes the pitch of the concave mirrorsconstituting the mirror array 200. A ratio Pm/Pg is set to an integer sothat the dark portion of the grating 30 confronts the boundary portionbetween two mutually adjacent concave mirrors of the mirror array 200.For example, Pm/Pg=2 in the present embodiment. According to the presentembodiment, it is possible to obtain substantially the same effects asthose obtainable in the fifth and sixth embodiments, and the performanceof the synchronizing signal generating system is greatly improvedcompared to that of the conventional system using the mirror array.

In a fifteenth embodiment of the synchronizing signal generating systemaccording to the present invention, the converging optical system 32C isreplaced by a mirror array comprising concave mirrors in the seventhembodiment shown in FIG. 16 described before. FIG. 49 shows in a planview the relationship of the grating 30, the converging optical systemand the light receiving system. The remaining parts of the recordingapparatus or document reading apparatus may be the same as those of theembodiments described heretofore, and an illustration and descriptionthereof will be omitted.

In the present embodiment, a converging optical system 210 comprisesconcave mirrors which are arranged along a curve as shown, so that thesynchronizing beam transmitted through the grating 30 hit centers of theconcave mirrors. As a result, it is possible to satisfactorily convergethe synchronizing beam, and the converged beam spot caused by theaberration is small. For this reason, a light receiving element having asmall light receiving area and operable at a high speed may be used forthe light receiving elements of a light receiving system 211. Therefore,the present embodiment can be applied to a high speed recordingapparatus or document reading apparatus, and it is possible to obtainsubstantially the same effects as those obtainable in the seventhembodiment.

It is readily apparent that the fifteenth embodiment may be modified sothat the concave mirrors of the converging optical system are arrangedgenerally along a line as in the case of the eighth embodiment shown inFIG. 17 described before.

Although illustration and description thereof will be omitted forconvenience' sake, the tenth through thirteenth embodiments forelectrically correcting the synchronizing clock signal may be similarlyapplied to the case where the converging optical system comprises themirror array as in the case of the fourteenth and fifteenth embodiments.Substantially the same effects are obtainable, thereby considerablyimproving the performance compared to the conventional system using themirror array.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A synchronizing signal generating system for alaser scanner, said synchronizing signal generating systemcomprising:first means for emitting a first laser beam for scanning anda second laser beam for synchronization of scans by said first laserbeam; second means for scanning a medium by said first laser beam; thirdmeans for scanning a grating by said second laser beam, said gratinghaving bright portions and dark portions alternately arranged along ascanning direction of said second laser beam; a converging opticalsystem for converging said second laser beam transmitted through saidgrating; a light receiving system for receiving said second laser beamconverged by said converting optical system and for generating asynchronizing signal; and fourth means for controlling said first meansresponsive to said synchronizing signal so as to synchronize a scantiming of said first laser beam, said converging optical systemcomprising a plurality of lenses provided for a length corresponding toa scan length of said second laser beam on said grating.
 2. Asynchronizing signal generating system as claimed in claim 1 in whichsaid lenses constitute a lens array, said lens array comprising thelenses successively arranged side by side along a scanning direction ofsaid second laser beam for a full width of said scan length of saidsecond laser beam.
 3. A synchronizing signal generating system asclaimed in claim 1 in which said light receiving system comprises aplurality of light receiving elements for receiving converged light fromcorresponding ones of said lenses of said converging optical system. 4.A synchronizing signal generating system as claimed in claim 1 in whichsaid light receiving system comprises a plurality of optical fibershaving receiving ends for receiving converged light from correspondingones of said lenses of said converging optical system, converging meansfor converging light emitted from emitting ends of said optical fibers,and a light receiving element for receiving converged light from saidconverging means.
 5. A synchronizing signal generating system as claimedin claim 1 in which said lenses are convex lenses which constitute alens array, said lens array comprising the convex lenses successivelyarranged side by side along a scanning direction of said second laserbeam for a full width of said scan length of said second laser beam. 6.A synchronizing signal generating system as claimed in claim 5 in whicha ratio P1/Pg between a pitch P1 of said convex lenses and a pitch Pg ofthe bright and dark portions of said grating is set to an integer, sothat said second laser beam transmitted through said grating isprevented from scanning a junction portion between two mutually adjacentconvex lenses of said lens array.
 7. A synchronizing signal generatingsystem as claimed in claim 6 in which said lens array comprises a flatsurface and a curved surface defined by lens surfaces of the convexlenses, said flat surface facing said grating.
 8. A synchronizing signalgenerating system as claimed in claim 5 in which a ratio P1/Pg between apitch P1 of said convex lenses and a pitch of the bright and darkportions of said grating is slightly greater than an integer, so thatsaid second laser beam transmitted through said grating is preventedfrom scanning a junction portion between two mutually adjacent convexlenses of said lens array.
 9. A synchronizing signal generating systemas claimed in claim 8 in which said lens array comprises a flat surfaceand a curved surface defined by lens surfaces of the convex lenses, saidcurved surface facing said grating.
 10. A synchronizing signalgenerating system as claimed in claim 1 in which said first meanscomprises a first laser source for emitting said first laser beam and asecond laser source for emitting said second laser beam.
 11. Asynchronizing signal generating system as claimed in claim 1 in whichsaid first means comprises a single laser source for emitting a laserbeam, a semitransparent mirror for reflecting a portion of said laserbeam as said first laser beam, and a mirror for reflecting a portion ofsaid laser beam transmitted through said semitransparent mirror as saidsecond laser beam.
 12. A synchronizing signal generating system asclaimed in claim 1 in which said lenses are convex lenses whichconstitute a lens array, said lens array comprising the convex lensessuccessively arranged side by side generally along a scanning directionof said second laser beam for a full width of said scan length of saidsecond laser beam so that said second laser beam transmitted throughsaid grating and reaching centers of said convex lenses approximatelycoincide with respective optical axes of said convex lenses.
 13. Asynchronizing signal generating system as claimed in claim 1 in whichsaid lenses are convex lenses arranged along a scanning direction ofsaid second laser beam at predetermined intervals.
 14. A synchronizingsignal generating system as claimed in claim 13 in which said thirdmeans comprises an fθ-lens through which said second laser beam reachessaid grating, said convex lenses being arranged within predeterminedranges of said grating so as to include inflection points of a linearitycharacteristic of a scanning position of said synchronizing beamtransmitted through said fθ-lens with respect to a synchronizing beamangle θ.
 15. A synchronizing signal generating system as claimed inclaim 14 which further comprises means for blocking said grating withinranges other than said predetermined ranges.
 16. A synchronizing signalgenerating system as claimed in claim 1 in which said fourth meanscomprises a phase locked loop circuit for generating an image clocksignal responsive to said synchronizing signal, said image clock signalhaving a frequency higher than that of said synchronizing signal, anddriving means for driving said first means in synchronism with saidimage clock signal depending on input information data.
 17. Asynchronizing signal generating system as claimed in claim 1 whichfurther comprises signal generating means for generating a pseudosynchronizing signal with a timing corresponding to a joint portionbetween two mutually adjacent lenses of said converging optical system,and selector means for selectively outputting said synchronizing signaland said pseudo synchronizing signal so as to output a continuoussynchronizing signal having a stable duty cycle, said fourth meanscomprising a phase locked loop circuit for generating an image clocksignal responsive to said continuous synchronizing signal, said imageclock signal having a frequency higher than that of said synchronizingsignal, and driving means for driving said first means in synchronismwith said image clock signal depending on input information data.
 18. Asynchronizing signal generating system as claimed in claim 1 whichfurther comprises correcting means for correcting a duty cycle of saidsynchronizing signal, said fourth means comprising a phase locked loopcircuit for generating an image clock signal responsive to a correctedsynchronizing signal obtained from said correcting means, said imageclock signal having a frequency higher than that of said synchronizingsignal, and driving means for driving said first means in synchronismwith said image clock signal depending on input information data.
 19. Asynchronizing signal generating system as claimed in claim 18 in whichsaid correcting means comprises an edge detector for detecting a risingedge of said synchronizing signal and a pulse producing circuit forproducing an output pulse having a constant pulse width from the risingedge detected by said edge detector, said output pulse of said pulseproducing circuit being outputted as said corrected synchronizingsignal.
 20. A synchronizing signal generating system as claimed in claim1 in which said synchronizing signal is generated during a scan durationin which said medium is scanned by said first laser beam and nosynchronizing signal is generated during a non-scan duration in which noscan of said medium is made by said first laser beam, said synchronizingsignal generating system further comprising signal generating means forgenerating a pseudo synchronizing signal during said non-scan duration,and selector means for selectively outputting said synchronizing signaland said pseudo synchronizing signal so as to output a continuoussynchronizing signal, said pseudo synchronizing signal having apredetermined frequency identical to that of said synchronizing signal.21. A synchronizing signal generating system as claimed in claim 20 inwhich said predetermined frequency is an integral multiple of a scanningfrequency of said first laser beam with respect to said medium.
 22. Asynchronizing signal generating system as claimed in claim 20 in whichsaid signal generating means generates said pseudo synchronizing signalhaving a phase which coincides with that of said synchronizing signal.23. A synchronizing signal generating system as claimed in claim 22 inwhich said signal generating means comprises a detector for detecting atiming of each scan duration, and circuit means for generating saidpseudo synchronizing signal with the timing detected by said detector.24. A synchronizing signal generating system as claimed in claim 23 inwhich said second means comprises a polygonal mirror which rotates anddeflects said first laser beam so as to scan said medium, said detectorcomprising means for detecting a rotation frequency of said polygonalmirror.
 25. A synchronizing signal generating system as claimed in claim20 in which said fourth means comprises a phase locked loop circuit forgenerating an image clock signal responsive to said continuoussynchronizing signal from said selector means, said image clock signalhaving a frequency higher than that of said synchronizing signal, anddriving means for driving said first means in synchronism with saidimage clock signal depending on input information data.
 26. Asynchronizing signal generating system as claimed in claim 1 in whichsaid second means scans said medium to record information thereon.
 27. Asynchronizing signal generating system as claimed in claim 1 in whichsaid second means scans said medium to read information therefrom.
 28. Asynchronizing signal generating system as claimed in claim 1 in whichsaid second means comprises a polygonal mirror which rotates anddeflects said first laser beam so as to scan said medium, and an fθ-lensfor transmitting said first laser beam from said polygonal mirror, saidsecond means comprising a mirror for reflecting said first laser beamfrom said fθ-lens onto said medium.
 29. A synchronizing signalgenerating system as claimed in claim 1 in which said second meanscomprises a polygonal mirror which rotates and deflects said first laserbeam so as to scan said medium, and an fθ-lens for transmitting saidfirst laser beam from said polygonal mirror, said third means comprisinga mirror for reflecting said second laser beam from said fθ-lens ontosaid grating.
 30. A synchronizing signal generating system for a laserscanner, said synchronizing signal generating system comprising:firstmeans for emitting a first laser beam for scanning and a second laserbeam for synchronization of scans by said first laser beam; second meansfor scanning a medium by said first laser beam; third means for scanninga grating by said second laser beam, said grating having bright portionsand dark portions alternately arranged along a scanning direction ofsaid second laser beam; a converging optical system for converging saidsecond laser beam transmitted through said grating; a light receivingsystem for receiving said second laser beam converged by said convertingoptical system and for generating a synchronizing signal; and fourthmeans for controlling said first means responsive to said synchronizingsignal so as to synchronize a scan timing of said first laser beam, saidconverging optical system comprising a mirror array made up of aplurality of concave mirrors, said mirror array being provided for alength corresponding to a scan length of said second laser beam on saidgrating, a ratio Pm/Pg between a pitch Pm of said concave mirrors and apitch Pg of the bright and dark portions of said grating being set to aninteger, so that said second laser beam transmitted through said gratingis prevented from scanning a junction portion between two mutuallyadjacent concave mirrors of said mirror array.
 31. A synchronizingsignal generating system as claimed in claim 30 in which said lightreceiving system comprises a plurality of light receiving elements forreceiving converged light from corresponding ones of said concavemirrors of said converging optical system.
 32. A synchronizing signalgenerating system as claimed in claim 30 in which said light receivingsystem comprises a plurality of optical fibers having receiving ends forreceiving converged light from corresponding ones of said concavemirrors of said converging optical system, converging means forconverging light emitted from emitting ends of said optical fibers, anda light receiving element for receiving converged light from saidconverging means.
 33. A synchronizing signal generating system asclaimed in claim 30 in which said first means comprises a first lasersource for emitting said first laser beam and a second laser source foremitting said second laser beam.
 34. A synchronizing signal generatingsystem as claimed in claim 30 in which said first means comprises asingle laser source for emitting a laser beam, a semitransparent mirrorfor reflecting a portion of said laser beam as said first laser beam,and a mirror for reflecting a portion of said laser beam transmittedthrough said semitransparent mirror as said second laser beam.
 35. Asynchronizing signal generating system as claimed in claim 30 in whichsaid second means scans said medium to record information thereon.
 36. Asynchronizing signal generating system as claimed in claim 30 in whichsaid second means scans said medium to read information therefrom.
 37. Asynchronizing signal generating system as claimed in claim 30 in whichsaid second means comprises a polygonal mirror which rotates anddeflects said first laser beam so as to scan said medium, and an fθ-lensfor transmitting said first laser beam from said polygonal mirror, saidsecond means comprising a mirror for reflecting said first laser beamfrom said fθ-lens onto said medium.
 38. A synchronizing signalgenerating system as claimed in claim 30 in which said second meanscomprises a polygonal mirror which rotates and deflects said first laserbeam so as to scan said medium, and an fθ-lens for transmitting saidfirst laser beam from said polygonal mirror, said third means comprisinga mirror for reflecting said second laser beam from said fθ-lens ontosaid grating.
 39. A synchronizing signal generating system for a laserscanner, said synchronizing signal generating system comprising:firstmeans for emitting a first laser beam for scanning and a second laserbeam for synchronization of scans by said first laser beam; second meansfor scanning a medium by said first laser beam; third means for scanninga grating by said second laser beam, said grating having bright portionsand dark portions alternately arranged along a scanning direction ofsaid second laser beam; a converging optical system for converging saidsecond laser beam transmitted through said grating; a light receivingsystem for receiving said second laser beam converged by said convertingoptical system and for generating a synchronizing signal; and fourthmeans for controlling said first means responsive to said synchronizingsignal so as to synchronize a scan timing of said first laser beam, saidconverging optical system comprising a mirror array made up of aplurality of concave mirrors, said mirror array being provided for alength corresponding to a scan length of said second laser beam on saidgrating, a ratio Pm/Pg between a pitch Pm of said concave mirrors and apitch of the bright and dark portions of said grating being slightlygreater than an integer, so that said second laser beam transmittedthrough said grating is prevented from scanning a junction portionbetween two mutually adjacent concave mirrors of said mirror array. 40.A synchronizing signal generating system as claimed in claim 39 in whichsaid light receiving system comprises a plurality of light receivingelements for receiving converged light from corresponding ones of saidconcave mirrors of said converging optical system.
 41. A synchronizingsignal generating system as claimed in claim 39 in which said lightreceiving system comprises a plurality of optical fibers havingreceiving ends for receiving converged light from corresponding ones ofsaid concave mirrors of said converging optical system, converging meansfor converging light emitted from emitting ends of said optical fibers,and a light receiving element for receiving converged light from saidconverging means.
 42. A synchronizing signal generating system asclaimed in claim 39 in which said first means comprises a first lasersource for emitting said first laser beam and a second laser source foremitting said second laser beam.
 43. A synchronizing signal generatingsystem as claimed in claim 39 in which said first means comprises asingle laser source for emitting a laser beam, a semitransparent mirrorfor reflecting a portion of said laser beam as said first laser beam,and a mirror for reflecting a portion of said laser beam transmittedthrough said semitransparent mirror as said second laser beam.
 44. Asynchronizing signal generating system for a laser scanner, saidsynchronizing signal generating system comprising:first means foremitting a first laser beam for scanning and a second laser beam forsynchronization of scans by said first laser beam; second means forscanning a medium by said first laser beam; third means for scanning agrating by said second laser beam, said grating having bright portionsand dark portions alternately arranged along a scanning direction ofsaid second laser beam; a converging optical system for converging saidsecond laser beam transmitted through said grating; a light receivingsystem for receiving said second laser beam converged by said convertingoptical system and for generating a synchronizing signal; fourth meansfor controlling said first means responsive to said synchronizing signalso as to synchronize a scan timing of said first laser beam; and signalgenerating means for generating a pseudo synchronizing signal, saidconverging optical system comprising a mirror array made up of aplurality of concave mirrors, said mirror array being provided for alength corresponding to a scan length of said second laser beam on saidgrating, said fourth means comprising a phase locked loop circuit forgenerating an image clock signal responsive to said synchronizingsignal, said image clock signal having a frequency higher than that ofsaid synchronizing signal, and driving means for driving said firstmeans in synchronism with said image clock signal depending on inputinformation data.
 45. A synchronizing signal generating system asclaimed in claim 44 in which said signal generating means generates saidpseudo synchronizing signal with a timing corresponding to a boundaryportion between two mutually adjacent concave mirrors of said convergingoptical system, and said synchronizing signal generating system furthercomprises selector means for selectively outputting said synchronizingsignal and said pseudo synchronizing signal so as to output a continuoussynchronizing signal having a stable duty cycle, said continuoussynchronizing signal being supplied to said phase locked loop circuit assaid synchronizing signal.
 46. A synchronizing signal generating systemfor a laser scanner, said synchronizing signal generating systemcomprising:first means for emitting a first laser beam for scanning anda second laser beam for synchronization of scans by said first laserbeam; second means for scanning a medium by said first laser beam; thirdmeans for scanning a grating by said second laser beam, said gratinghaving bright portions and dark portions alternately arranged along ascanning direction of said second laser beam; a converging opticalsystem for converging said second laser beam transmitted through saidgrating; a light receiving system for receiving said second laser beamconverged by said converting optical system and for generating asynchronizing signal; fourth means for controlling said first meansresponsive to said synchronizing signal so as to synchronize a scantiming of said first laser beam; and correcting means for correcting aduty cycle of said synchronizing signal and for outputting a correctedsynchronizing signal, said converging optical system comprising a mirrorarray made up of a plurality of concave mirrors, said mirror array beingprovided for a length corresponding to a scan length of said secondlaser beam on said grating, said fourth means comprising a phase lockedloop circuit for generating an image clock signal responsive to saidcorrected synchronizing signal, said image clock signal having afrequency higher than that of said synchronizing signal, and drivingmeans for driving said first means in synchronism with said image clocksignal depending on input information data.
 47. A synchronizing signalgenerating system as claimed in claim 46 in which said correcting meanscomprises an edge detector for detecting a rising edge of saidsynchronizing signal and a pulse producing circuit for producing anoutput pulse having a constant pulse width from the rising edge detectedby said edge detector, said output pulse of said pulse producing circuitbeing outputted as said corrected synchronizing signal.
 48. Asynchronizing signal generating system or a laser scanner, saidsynchronizing signal generating system comprising:first means foremitting a first laser beam for scanning and a second laser beam forsynchronization of scans by said first laser beam; second means forscanning a medium by said first laser beam; third means for scanning agrating by said second laser beam, said grating having bright portionsand dark portions alternately arranged along a scanning direction ofsaid second laser beam; a converging optical system for converging saidsecond laser beam transmitted through said grating; a light receivingsystem for receiving said second laser beam converged by said convertingoptical system and for generating a synchronizing signal; fourth meansfor controlling said first means responsive to said synchronizing signalso as to synchronize a scan timing of said first laser beam; signalgenerating means for generating a pseudo synchronizing signal; andselector means for selectively outputting one of said synchronizingsignal and said pseudo synchronizing signal so as to output a continuoussynchronizing signal, said converging optical system comprising a mirrorarray made up of a plurality of concave mirrors, said mirror array beingprovided for a length corresponding to a scan length of said secondlaser beam on said grating, said fourth means comprising a phase lockedloop circuit for generating an image clock signal responsive to saidcontinuous synchronizing signal, said image clock signal having afrequency higher than that of said synchronizing signal, and drivingmeans for driving said first means in synchronism with said image clocksignal depending on input information data, said synchronizing signalbeing generated during a scan duration in which said medium scanned bysaid first laser beam and no synchronizing signal being generated duringa non-scan duration in which no scan of said medium is made by saidfirst laser beam, said signal generating means generating said pseudosynchronizing signal during said non-scan duration, said pseudosynchronizing signal having a predetermined frequency identical to thatof said synchronizing signal.
 49. A synchronizing signal generatingsystem as claimed in claim 48 in which said predetermined frequency isan integral multiple of a scanning frequency of said first laser beamwith respect to said medium.
 50. A synchronizing signal generatingsystem as claimed in claim 48 in which said signal generating meansgenerates said pseudo synchronizing signal having a phase whichcoincides with that of said synchronizing signal.
 51. A synchronizingsignal generating system as claimed in claim 50 in which said signalgenerating means comprises a detector for detecting a timing of eachscan duration, and circuit means for generating said pseudosynchronizing signal with the timing detected by said detector.
 52. Asynchronizing signal generating system as claimed in claim 51 in whichsaid second means comprises a polygonal mirror which rotates anddeflects said first laser beam so as to scan said medium, said detectorcomprising means for detecting a rotation frequency of said polygonalmirror.